The invention relates to a tire bead ring made of round wire disposed in the shape of a parallelogram and a tire made incorporating such a bead.
In the tire art, many devices and tire modifications have been developed for the purpose of maintaining air pressure in an inflated pneumatic tire. The main non-catastrophic cause of air loss in a tire is an incomplete or intermittent seal between the tire bead and the rim. To insure a good seal between the tire bead and the rim, the prior art has used various toe guards, chafers, specially shaped rims, and other complex sealing devices comprising special materials in constructing a tire. It has been found that one of the reasons that it is difficult to maintain a good seal between the tire and the rim is that the bead has a tendency to move or rock (rotate off the rim) when the tire is inflated or when it is subjected to the dynamic stresses encountered during running.
It is believed that bead rocking also contributes to ply separation in the bead area, either because bead rocking causes stresses on the plies, or because the same stresses that cause bead rocking contribute to ply separation.
It has been proposed that by improving the torsional rigidity of the bead, that the bead rocking phenomenon can be reduced and substantially minimized. For this reason, the prior art has adopted the use of hexagonal beads. It is believed that the improved torsional rigidity of a hexagonal bead limits the rocking of the bead area of the tire when compared to a tire made using a round bead. Even so, using a prior art hexagonal bead ring as an example, when a carcass ply wraps around such a bead ring, it makes substantial contact with the bead at four of its six corners. Contact with two of the six corners (the two outside corners) provides the major lifting forces encountered by the bead area of the tire. The vectors of the forces acting at these two points act together to provide a twisting couple which tends to cause the bead area to rotate on the rim.
One method the prior art has used to prevent twisting couples from acting on a bead ring is to provide a square bead having a 0.degree. lay angle, wherein major lifting forces are encountered only at one corner of the bead. Such a square bead is not suitable for all applications.
In some prior art applications, it has been suggested that flat wires may be used in bead rings to prevent deformation of the bead ring during vulcanization, which deformation results in different strands of bead wire being under different tensions. See, for example U.S. Pat. Nos. 3,949,800 and 4,290,471. It has been found, however, that it is difficult to obtain flat wires in a desired shape within tolerances (see the '471 patent, col. 1, lines 40-46) and accordingly, are sometimes difficult to manufacture as desired. Also, when flat wires are used and stacked, forces applied to one of the flat wires are translated to adjacent flat wires undiminished, and the entire bead ring is substantially affected by such forces.
The bead rocking or toe lifting phenomenon is related to bead twisting.
In accordance with the present invention, it has been theorized that if the bead rocking phenomenon is minimized, and if the twisting forces encountered by the bead can be minimized, that less stringent methods can be used to ensure that a seal is maintained between the tire bead and the rim, reducing the labor and consequently the expense of building a tire.
Also, it is expected that if twisting forces encountered by a tire bead are minimized, either as a result of reduced bead rocking or in a way that reduces bead rocking, the added stability of the bead will cause a reduction of ply separation at the ply ending in the bead area of the tire and reduce wear (such as chafing) and improve bead durability.
It is an object of the present invention to provide a tire bead ring which is easy to build and which dissipates twisting forces encountered by a tire bead and has a shape and disposition which is less susceptible to bead rocking and toe lifting because the torques encountered by the bead are minimized as compared to conventional hexagonal beads.
It is also an object of the invention to provide a tire incorporating the bead of the invention.
Other objects of the invention will be apparent from the following description and claims.